Most cooling systems, particularly vehicle engine cooling systems, use water as the primary cooling medium because of its good heat transfer qualities, low cost and universal availability. It has two disadvantages which, however, are of particular interest to its use as a cooling medium for internal combustion engines: (a) it has a relatively high freezing point, and (b) a tendency to corrode metals used in the engine cooling system. To prevent damage to the metallic components of the system by frozen coolant, a freezing point depressant is added in the form of an antifreeze solution.
Modern antifreeze solutions are also formulated to contain corrosion inhibitors for the various metals used in a cooling system such as copper, brass, cast iron, steel, solder, aluminum and zinc. It is becoming more prevalent for some components of the cooling system now to be made of aluminum alloys. Corrosion of aluminum or aluminum alloys by heat transfer is of principal concern here; heat transfer corrosion is used to mean corrosion of a metal at the metal/solution interface, which results solely from heat being transferred through the metal and rejected to the solution. Heat transfer corrosion of aluminum alloys is particulary troublesome because such alloys usually operate at higher interface temperatures with the cooling fluid tending to promote heat transfer corrosion. A change in the chemistry of the cooling solution over long usage also increases heat transfer corrosion. Heat transfer corrosion can produce either actual perforation of a metal wall or, more frequently, the clogging of heat exchanger tubes with corrosion products.
Both organic and inorganic compounds have been used as corrosion inhibitors added to antifreeze solutions. Organic compounds have comprised mineral and vegetable oils, and their sulfonated products, phosphonates, amines, amides, triazoles, benzoates and mercaptans; complex organosilicon compounds such as silicones, silanes and siloxanes have also been used. Inorganic compounds have included silicates, borates, phosphates, nitrates and nitrites, molybdates, arsenites and tellurites. Most of these corrosion inhibitors are not effective in protecting aluminum based metals of a cooling system and therefore are ineffective in solving the problem of this invention. Those that have shown some degree of effectiveness in protecting aluminum include phosphates, nitrates and silicates. However, the prior art has viewed the corrosion problem as one which can be solved by a short-term solution which includes adding various corrosion inhibitors to the premixed solution. Such approach overlooks the change in the chemistry of the cooling solution that takes place over long-term use.
In the European and Japanese automobile industry, it has been prevalent to use antifreeze solutions having corrosion inhibitors comprised either of (a) benzoate/borate/nitrite aggregation, which has proved to be relatively weak in the protection of aluminum, or (b) the use of a nitrate in combination with triethanolamine phosphate. To eliminate the heat transfer corrosion with nitrates, the Europeans have added triethanolamine phosphate. Unfortunately, the amine inhibitor can be nitrosated to form nitrosamine; nitrosamines have been shown to be carcinogenic in laboratory animals and thus are avoided in the United States.
In the United States, alkali metal tetraborates and phosphates in combination with silicates have become prevalent ingredients in antifreeze solutions. Both borates and phosphates tend to cause unwanted heat transfer corrosion products with aluminum, particularly very hot surfaces of aluminum cylinder heads. Phosphates additionally suffer from a moderately fast depletion rate in use. Although silicates have been used in combination with tetraborates and phosphates, they have not proved entirely successful in preventing extended heat transfer corrosion because silicates exhibit an even faster depletion rate.